The present invention generally relates to an aircraft having a wing free for rotation about a spanwise axis to maintain a constant angle of attack with the relative wind. More particularly, the present invention relates to a wing which is lockable and unlockable relative to the fuselage to selectively enable free rotation of the wing about the spanwise axis, and hence flight in a free wing mode, and fixation of the wing to the fuselage at a predetermined angle of incidence, and hence flight in a conventional fixed wing mode.
A free wing or "freewing" is a wing attached to an aircraft fuselage in a manner such that the wing is freely pivotable about its spanwise axis forward of its aerodynamic center. This arrangement enables the wing to have an angle of attack which is determined solely by aerodynamic forces, and therefore, subject only to aerodynamic pitching moments imposed by wing lift and drag. Rotation of the wing, without pilot intervention, induced by positive or negative vertical wind gusts striking the wing causes the angle of incidence or pitch between the wing and the aircraft fuselage to vary so that the wing presents a constant angle of attack to the relative wind enabling the aircraft to be essentially stall-free during flight.
Among other advantages realized when employing a freewing are increased resistance to stalls, increased C.G. (Center of Gravity) range, alleviation of gust loads, e.g. on the order of a 4:1 reduction, which translates to an increase in passenger comfort of up to 4,000% greater than conventional fixed wing aircraft, extension of the payload capability due to the ability to reduce the structural weight of the aircraft, and the ability to utilize a smaller engine with a lower fuel requirement, thus increasing the flight range of the aircraft.
Recognizing the advantages associated with freewings, numerous attempts have been made to adapt the concept of the freewing to conventional aircraft, particularly in the field of general aviation. However, as discussed below, the freewing concept has only been successfully applied to light or very light aircraft.
U.S. Pat. No. 4,596,368, issued to the present inventor is directed to an ultra-light aircraft wherein a hang cage is suspended from a collapsible Rogallo type wing by a main hinge assembly. The wing includes a longitudinal keel of light-weight tubular construction, leading edge members, and a cross brace. A flexible lifting panel is secured along and between these members to establish a lifting surface. The hinge is clamped to the keel permitting free rotation of the wing about a spanwise axis extending longitudinally through the cross brace.
U.S. Pat. No. 4,568,043, also issued to the present inventor, is directed to an ultra-light aircraft of light-weight construction which includes a freely rotating rigid wing from which a hang cage is suspended by a main hinge assembly
Although the freewing concept has been successfully applied to ultra-light aircraft by the present inventor and to a light plane water-borne aircraft, the application of the freewing concept to conventional aircraft, particularly in the field of general aviation, has not been successfully achieved due to the fact that not only is drag increased as a consequence of the flap deflection needed to properly vary the coefficient of lift of the wing, but in addition, deploying flaps on a freewing can induce instability of the wing by changing the pitching moment of the airfoil from a positive value, needed for stable freewing flight, to a negative value. This characteristic of freewings becomes particularly significant when an aircraft is on final approach and/or during takeoff, as can readily be appreciated by those familiar with the handling of aircraft in flight.
Aircraft such as ultra-lights can ignore the problem associated with low wing lift coefficient, because they are inherently so low wing loaded that they can fly at speeds slow enough to take off and land safely. However, without some method of inducing a freewing into higher lift coefficients than possible in the pure freewing, larger size freewing aircraft are not easily attainable.
Solutions proposed and explored in NASA-funded studies include (i) using leading edge slats instead of trailing edge flaps and (ii) using a freewing free-trimmer, a device resembling a canard surface protruding from the wing ahead of the leading edge, supported by booms, as well as an alternative version of the freewing free-trimmer, in which the control surfaces were moved from the canard position to a trailing edge location.
The proposed use of leading edge slats was determined to be structurally over-complicated and otherwise did not provide the required lift coefficient additive. The proposed use of a freewing free-trimmer was the subject of two further NASA-funded studies. In one of these further studies, NASA CR-2946, Analytical Study of a Free-Wing/Free-Trimmer Concept (1978), it was found that while there was some additional lift force possible with aft mounted trimmers, they required additional mass penalty to balance the freewing about its hinge line. Moreover, it was found that while forward mounted (canard) trimmers would themselves serve as the required counterweight, they also counteracted the gust alleviation qualities of the freewing to the extent that such a freewing would provide an even rougher ride in turbulence than its conventional counterpart, thus destroying one of the desired benefits of the freewing.
A number of U.S. patents involving freewing and freewing-like designs have issued. For example, U.S. Pat. No. Re. 18,181 to Stelzer discloses a type of freewing which includes a means to provide elastic shock absorption.
U.S. Pat. No. 3,415,469 to Spratt, builder of the aforementioned aircraft, discloses a freewing type aircraft in which the pitch of the wing is controlled by the pilot through control stick 46 or control wing 31. In operation, control rods 42 and 43 impart a torque about the wing hinges such that the wing assumes a higher angle of attack while still remaining largely free to rotate in response to gusts.
U.S. Pat. No. 4,124,180 to Wolowicz discloses a freewing aircraft which incorporates a trimmable free stabilizer comprising a floating canard mounted on a strut rigidly connected to the freewing.
The present invention involves a revolutionary new approach to the problem of how to incorporate the freewing concept to aircraft of any size.